North Korea has warned South Korea that its participation in UN sanctions against Pyongyang would be seen as a serious provocation.
North Korea said the South could pay dearly for imposing sanctions
A spokesman for the country's Committee for the Peaceful Reunification warned it would lead to a "crisis of war".
The comments were carried by North Korea's official news agency, KCNA.
Arms and financial sanctions were unanimously approved by the UN Security Council following North Korea's nuclear test on 9 October.
"If the South Korean authorities end up joining US-led moves to sanction and stifle, we will regard it as a declaration of confrontation against its own people ... and take corresponding measures," the statement said.
The UN Security Council voted on 14 October to impose financial and arms sanctions on North Korea after it conducted a nuclear test. It calls on all members to state how they plan to implement sanctions by mid-November.
South Korea has formed a task force to look at how to impose sanctions, which held its first meeting on Tuesday.
But Russian President Vladimir Putin has said North Korea should not be driven into a corner if the world wants to rein in its nuclear weapons programme.
He said some negotiators "failed to find the right tone" with the country, adding a solution could be found "with goodwill".
Meanwhile, South Korea's Unification Minister Lee Jong-seok has offered his resignation over his country's handling of North Korea's nuclear test.
Mr Lee was criticised for not having tough enough policies against Pyongyang.
"During this political strife, I thought someone with more talent than I have should come to this position and overcome the problem," he said.
It comes a day after the South Korean defence minister, Yoon Kwang-ung, tendered his resignation.
Japan is also debating its handling of the nuclear crisis.
The country's defence chief, Fumio Kyuma, has said the country should not engage in "careless debate" over whether it should possess nuclear weapons.
His comments were in reaction to those made by politicians including Foreign Minister Taro Aso, suggesting Japan discuss the development of nuclear warheads.
"We have advanced technology and missile capabilities so perhaps we do have the potential to make nuclear arms. But we are not going to do so," he said.